There exist devices on the market today for interfacing a plurality of PC-type computers each having its own power supply to a single keyboard and monitor. One well-known device of this type is the COMMANDER, manufactured by Cybex Corporation of Huntsville, Ala. In this device, up to 96 microcomputers may be connected to the COMMANDER module and accessed by the keyboard and monitor. Coupled to the COMMANDER module is a single keyboard and monitor, with the keyboard and monitor being selectively coupled via the module to any one of the computers connected to the module. Thus, the COMMANDER module, in addition to other functions, interfaces the single keyboard and monitor to one of several computers, any one of which may be powered "on" or "off" at any point in time.
Uses of these devices occur generally where it is desired to infrequently access one of a number of computers via a keyboard and monitor, each performing a separate function and being grouped together at a single location, such as in a control room of a large building wherein separate computers control heating systems, elevators, lighting, etc. Another use occurs in the networking of computers wherein separate computers function as file servers that provide information to remote users. In these situations, the computers may be automatically powered up or down as needed and may be infrequently accessed at their location for maintenance or to obtain statistical data therefrom. In this situation, it is highly desirable to provide a single keyboard and monitor selectively couplable to each of the computers as needed.
Problems arise with these interface devices with respect to the supplying of power to the keyboard. Ideally, as any one of the computer power supplies is capable of powering the keyboard, the logical approach would be to simply couple the power supplies of the computers together in parallel so that at least one of the computers provides power to the keyboard. However, the slight differences and transient surges in voltage levels between the power supplies of the computers cause a condition known as "power supply contention," which can drive voltage regulators of the power supplies into uncontrolled oscillations, disabling or possibly "smoking" the contending power supplies. Additionally, if one of the computers is switched "off," the computer power supplies that are "on" will try to maintain power in the "off" computer.
To overcome power supply contention, one approach that has been utilized is to provide a separate power supply in the interface module for supplying power to the keyboard. However, this leads to increased cost and complexity of the interface module and does not advantageously utilize the computers, which are each designed to provide power to a keyboard.
Another approach to overcome the aforestated problem is to provide a diode or other semiconductor device in the output of each power supply, the power supplies connected in a parallel hook-up as described in order to effectively isolate the respective power supplies. However, with keyboards requiring relatively low potentials, such as +5 volts, the 700-millivolt diode drop across a silicon semiconductor junction pulls the +5-volt level supplied by the power supply down to approximately 4.3 volts which, in some cases, is not high enough to supply the keyboard. In some instances, the power supplies of the respective computers may be "tweaked" to offset the diode drop, but this is not desirable. Additionally, germanium devices, which only have a 200-millivolt voltage drop, may be used for isolation purposes, but these devices are expensive and have well-known temperature limitations that make them less suitable than silicon devices for this application.
Accordingly, it is an object of this invention to provide a circuit that may be used to isolate discrete power supplies coupled together and provide power from one of the power supplies to a common lead without introducing an unacceptable voltage drop in the supply voltage.